Diversity receiving techniques are techniques for improving the quality of communication by combining a plurality of received signals received by a plurality of receiving systems respectively. By the diversity receiving techniques, time-selective, space-selective, or frequency-selective fading (waveform distortion) can be compensated.
As combining methods in the diversity receiving techniques, for example, the following methods are known: a selection combining method in which a received signal having the highest signal level or SNR (signal-to-noise ratio) is selected among a plurality of received signals; an equal-gain combining method in which phases of a plurality of received signals are adjusted so as to make all of them the same phase and then the sum total of the received signals is outputted; and a maximal-ratio combining method in which phases and amplitudes of a plurality of received signals are adjusted and then the sum total of the received signals is outputted. In the maximal-ratio combining method, amplitude adjustment is executed by weighting the received signals so as to maximize the SNR of the combined output. These combining methods are disclosed in non-patent reference 1 indicated below, for example. Non-patent reference 1 discloses a technique for reducing thermal noise by the diversity effect obtained by combining the signals, by using the characteristic that thermal noise at different receiving antenna systems are mutually uncorrelated. Non-patent reference 1 also shows the diversity effect quantitatively.
In various nations of the world, digitization of broadcasting has been developed widely, and in parallel, it has become common for various receivers such as home television receivers, vehicle-mounted broadcast receivers, and mobile information terminals, to have a digital broadcast reception function. As reception styles have been diversified, in addition to a television broadcasting service and a radio broadcasting service, a new type of broadcasting services which is a combination of features of those two services have been started actually. This trend is expected to cause a surge in the number of digital broadcast channels.
Further, broadcast reception techniques have been increasingly sophisticated in recent years, and highly functional digital broadcast receivers have been spreading in a large scale. Some functions have already been put to practical use: an automatic selection function to select broadcast programs that match preferences of viewers, and functions of simultaneous multiple-channel reception and recording, for example. On the other hand, vehicle-mounted broadcast receivers have begun to utilize automatic reception-area switching techniques for seamless reception and techniques for stable broadcast reception in environments where reception is difficult. The sophistication of digital broadcast receivers for higher functions is expected in any form of reception.
In particular, the simultaneous multiple-channel reception function is one of indispensable techniques for sophisticating the reception function to keep up with diversifying both broadcasting and reception styles. Accordingly, a variety of reception schemes have been studied in recent years in relation to the simultaneous multiple-channel reception function.
Simultaneous multiple-channel reception can be implemented easily by incorporating tuners, the number of which is the same as the number of channels needed for simultaneous reception, in a digital broadcast receiver. Each of the tuners has an analog front-end unit that converts an RF (radio frequency) signal in a high-frequency band to a low-frequency signal in a lower frequency band (such as an IF signal in an intermediate frequency band) by using an oscillation signal generated by a local oscillator. The plurality of tuners concurrently output a plurality of low-frequency signals corresponding to the multiple channels. It is, however, uneconomical to incorporate the plurality of tuners into the digital broadcast receiver for the simultaneous multiple channel reception, since the number of analog components required in the analog front-end unit increases as the number of channels increases.
It has been suggested to incorporate a plurality of frequency converters, the number of which is the same as the number of channels needed for simultaneous reception, and a signal adder into a single tuner. Such type of tuner is disclosed in Japanese Patent Application Publication No. 2001-007780 (patent reference 1), for example.
In the tuner disclosed in patent reference 1, the plurality of frequency converters convert an RF signal to a plurality of IF (intermediate-frequency) signals corresponding to the plurality of channels, by using a plurality of oscillation signals having oscillation-frequencies which are different from one another. A signal adder adds the plurality of IF signals together to output them. Here, the oscillation-frequencies are adjusted so that the channels (frequency bands) of IF signal components do not overlap one another in the output frequency spectrum of the signal adder. Thus, the tuner can then process the plurality of IF signals obtained by frequency conversion as signals of a single channel, and the number of analog components can be reduced. Since the analog output of the signal adder can be converted to a digital signal by a single A/D converter, a cost advantage can be gained, for example.